This invention relates generally to apparatus and methodology useful in effecting medical diagnosis, and more specifically relates to systems and methodology utilizing ultrasonic techniques for such purposes.
Over the course of the last several decades ultrasonic technology has played an ever-increasing role in medical diagnostics. Such techniques find application in diagnosis of various medical ailments wherein it is useful to examine internal bodily organs with the objective of locating features or aspects of such organs which may be indicative of disease, abnormalities or so forth.
While early systems of the foregoing type included but limited capabilities and display functions, there have more recently come into use highly sophisticated devices which are capable of providing real time or recorded displays with excellent detail and good resolution of desired portions of the body being considered.
In a typical such device the transducer utilized with the system comprises a phased array consisting of a plurality of transducer elements arranged in compact linear fashion. Each transducer element is individually connected to a suitable transmitter and receiver and the transmitted pulses are so phased as to steer the emitted sound beam in the desired direction. Adjustable delays provided in each receiver channel enhance the reception from the same direction as the transmitted sound beam. By suitably controlling the time of the voltages applied to the transducer elements and by controlling the adjustable delays of the separate receiver channels, the beam can be steered to any desired angle of a fan-shaped sector. Operation of the steered array is such that a plurality of radial lines defining the fan-shaped sector are successively generated with a relatively high number of such radial lines--typically of the order of 128 such lines--being utilized in the course of generating the entire sector. The set of such lines is generated over a short period, typically of the order of 1/30th of a second, whereby the corresponding display on the system cathode ray tube (CRT) is a high resolution substantially real time image of the bodily portion being examined. Said visualization is, in the terminology of the present art, a so-called B-mode display, i.e., one wherein variations of the acoustical impedance of the tissues are translated into brightness variations on the CRT screen.
Details regarding the prior art signal processing techniques utilized in apparatus of the foregoing type in order to generate the mentioned fan-shaped sector image are set forth in a number of points in the prior art. Reference may usefully be had, for example, to U.S. Pat. No. 4,005,382 to William Beaver entitled "Signal Processor for Ultrasonic Imaging," which patent is assigned to the assignee of the present application.
One of the serious problems that has plagued prior art systems of the foregoing type arises from poor resolution produced where the bodily portion being examined is present in the "near field" of the transducer. Conventional transducers and transducer arrays thus utilize the full active areas of the transducer faces in order to obtain maximum directivity and signal strength. While this procedure yields the desired results at distances larger than D.sub.2 /4.lambda..sub.1 where D is the maximum linear dimension of the active transducer face and .lambda. is the wavelength of the largest spectral component in the medium propagating the signal; yet within this range the directivity of the received radiation pattern suffers, the pattern becomes very complicated, the angular resolution is degraded, artifacts generated become very complicated because single point echoes may give multiple presentations, and the range resolution suffers significantly because of the spread in time of arrival at various points on the transducer face of signals originating from any single point in the near field.
In the co-pending application of L. T. Zitelli et al., Ser. No. 817,394, filed July 20, 1977 now U.S. Pat. No. 4,161,121 issued July 17, 1979 for "Ultrasonic Imaging System," which application is assigned to the assignee of the instant application, a system is disclosed which obviates certain of the foregoing difficulties. In particular, the basic concept of such system is one of altering the linear dimensions of the transducer array during the reception of an echo train in such a way as to take advantage of a small transducer at close range and to increase the effective size of the transducer with range by switching receiving elements as a function of time. Thus, pursuant to the teaching of said Zitelli et al., which is sometimes referred to as "pseudo-dynamic focusing," the effective size of the transducer array (i.e., effectively the transducer array aperture) is made smaller when waves are transmitted to or received from objects in its near-field Fresnel region, and is made larger when waves are transmitted to or received from objects farther from the transducer in either the Fresnel region or in the far-field region of the larger transducer. In consequence, the effective beam size is made as small as possible for both regions.
Nextly, it may be noted that the process of "dynamic focusing" has been known for several years, having been reported in some detail (among other places) in an article by F. L. Thurston and O. T. VonRamm entitled "A New Ultrasonic Imaging Technique Employing Two-Dimensional Electronic Beam Steering," which article appeared at pages 249 ff of "Acoustical Holography," Vol. 5, ed. by Phillip S. Greene, Plenum Press, New York (1974). In the technique of dynamic focusing, the acoustic beam is not only steered by manipulation of relative time delays but further is in the received mode dynamically focused, in such manner that the electrically determined focal length of the array is swept outwards in synchronism with the increasing range of target echoes. This result is achieved by varying the relative time delays applied to the signals received by each transducer element in such a way that the effective focal length of the array corresponds with the instantaneous value of the range from which echoes are being received.
In many (if not most) systems utilizing dynamic focusing, the actual changes in focus are not effected in continuous fashion, which would involve an inordinately complicated and relatively expensive system. Rather, the change in focal length is carried out in stepped fashion. So long as one is interested in points which are relatively far from the transducer, i.e., in the far field thereof, the required changes in focus need not be effected too rapidly since the focal ranges (i.e., the region including the focal length wherein resolution is satisfactory) in the far field are relatively extended. As one moves, however, to the near field and into regions closer to the transducer, the focal ranges wherein adequate resolution is present, become of shorter and shorter axial extension, in consequence of which refocusing steps must occur closer and closer together. This in turn limits the usefulness of the dynamic focusing approach, by increasing the complexity and cost of applying such a system to apparatus of the type wherein it will be most useful.
In accordance with the foregoing, it may be regarded as an object of the present invention to provide an ultrasonic imaging system based upon use of a linear phased array transducer, wherein the advantageous aspects of dynamic focusing are achieved, yet without the complexity and cost which such a feature would otherwise require for its effective use at near field regions of the transducer array.
It is a further object of the present invention to provide an ultrasonic imaging system of the foregoing character wherein good and relatively constant resolution are achievable throughout the entire distance range of the instrument's operation, and yet by the use of relatively simple and inexpensive apparatus implementations.